1. Field of the Invention
This invention relates to the reduction in Rayleigh waves (ground roll) in land seismic exploration. More particularly, it pertains to Rayleigh wave reduction through the selection of appropriate amplitude outputs from vibrator sources, coupled with the selection of appropriate separations between the vibrator sources.
2. Description of the Prior Art
Rayleigh waves (ground roll) is a seismic source noise that has plagued reflection seismology work for years. This surface wave travels at roughly half the speed of the coincident pressure wave and generally arrives at the receiver array from the vibrator sources at about the same time as pressure wave reflections from moderate depth targets. The surface wave is a coherent noise source and therefore cannot be effectively filtered in the frequency domain without reducing the signal energy in the same bandwidth.
Two prior art systems of suppressing Rayleigh waves are receiver array placement and velocity filtering. The receiver array system requires additional geophones to be used in the spread. Seismic data collection systems in current use typically use 200 to 500 geophone groups. For in-line suppression of 10 dB over three octaves, six or more geophones are needed per group. For omni directional suppression of 10 dB over three octaves, 20 or more geophones are required per group. The deployment and proper positioning of large numbers of geophones is labor intensive. Also, as the number of geophones is increased, so is the probability of equipment failure.
The system of velocity filtering is an off-line processing technique which exploits the fact that pressure waves travel faster than Rayleigh waves. For good suppression using velocity filtering, good static information, (terrain elevation), fine spatial sampling, (close group spacing) is required and a smooth transition in the acoustic properties of the weathered layer form group-to-group (no amplitude) anomalies due to receiver coupling variations. Furthermore, velocity filtering requires that the resolution of the data acquisition system be high enough so that low-level pressure wave signals can be recovered from the raw signal which is contaminated by relatively high level surface wave noise.
A relatively new system is described and claimed in copending U.S. patent application Ser. No. 790,794, filed Oct. 24, 1985, a continuation application of Ser. No. 516,409 now abandoned--"Attenuation of Source-Generated Noise in a Seismic Exploration System on a Floating Ice Layer" filed on July 22, 1983 and assigned to the assignee of this invention. This described system creates a baffle between the seismic sources and receivers by cutting a slit in the ice. A reduction of source generated flexure waves on the order of 10 dB has been measured. The method requires that a slit on the order of a mile be cut in the ice, which means that additional heavy machinery be built, transported and maintained. The extension of this technique to land based operations is not practical from an environment impact standpoint and, of course, from the standpoint of the equipment required to cut a mile long trough in the earth.
The present invention may be used in conjunction with either the receiver array technique or the velocity filtering system to achieve higher signal to noise ratios. It may also be used in place of complex receiver arrays to achieve the same amount of suppression without requiring the deployment of large numbers of geophones. Because the amount of surface wave noise generated by the seismic vibrators is actually reduced in the direction of the geophone, the need for high resolution data acquisition is diminished. By monitoring the Rayleigh wave velocity at the source, the seismic vibrator array can be automatically adjusted locally to achieve the greatest reduction in source generated Rayleigh waves.